The field of this invention is fluid supply systems and the like.
Offshore wells typically require a plurality of subsea well control devices which are remotely operated using fluid supplied from the ocean surface. Three basic functions performed by these control devices are control of gate valves, control of downhole safety valves, and injection of freezing retardant fluids into the flowlines coming from the wellhead. When flow valves in the wellhead are first opened and gas is released, gas in the flowline expands and may cause freezing. The injection of freezing retardant fluids such as methanol or glycol into the flowlines eliminates or reduces such freezing.
Typically, the devices which perform the three control functions mentioned above each require an operating fluid supplied at a different pressure level. For example, the control device for controlling gate valves may require operating fluid at a pressure of 1500 to 300 p.s.i., and the device for controlling downhole safety valves may require operating fluid at pressures on the order of 6000 p.s.i. while fluid is injected to the flowlines at relatively high and variable pressures typically in the range from 2000 p.s.i. to 4700 p.s.i.
Because of the differing fluid pressure requirements for the various control devices, operating fluid was often supplied through separate lines to each different control apparatus. Providing three or more flowlines for individually supplying operating fluid to the respective control devices was, of course, exceedingly expensive and somewhat unreliable. Other known systems employed booster arrangements and pumping arrangements to elevate the pressure of the operating fluid to the various desired levels. However, the addition of these active pumping means added not only expense but also inherent unreliability to such systems.